How to Test a Three-Phase Motor With a Megger

Eric W. Thompson

Three-phase power, introduced by inventor Nikoa Tesla, uses a polyphase system of alternating current to supply power to large motors and motor control systems. It uses less conductor material for power transmission than other methods while also providing a constant stream of power through a unique phase-delay between three alternating currents. To measure the resistance in a three-phase motor, you will need a megohmmeter, otherwise known as a megger. Unlike an ohmmeter, a megger can measure high-voltage resistance values, which are generally the kinds of values you will encounter with three-phase motors.

A megger is able to measure high-voltage resistance values.

Step 1

Turn off the power supply to the motor, usually accomplished by switching the circuit breaker. It is recommended that you either disconnect the power cable from the breaker box completely after switching the breaker off, or else have an assistant stand by and watch that no one flips the circuit breaker while you are megging the motor. If you have no one to watch the circuit breaker, be sure to surround the breaker box in yellow caution tape, and tape the breaker over with red warning tape.

Step 2

Place one megger probe to the any mounting bolt on the breaker box to test for grounding continuity, then touch the other probe to a motor terminal.

Step 3

Crank the handle for about a minute, and notice the resistance reading. Any reading above 0.5 meg-ohms is generally good.

Step 4

Test inter-phase resistance by disconnecting the leads from each terminal with a flathead screwdriver, then touch any two terminals with the two probes. Crank the megger and notice the reading. Any reading over 1 meg-ohm is sufficient.